The present invention relates generally to fiber optic directional couplers, and particularly to fiber optic directional couplers utilizing evanescent field coupling.
Fiber optic directional couplers transfer optical power between two fibers or strands of fiber optic material. In evanescent field coupling, guided modes of the two strands interact, through their evanescent fields, to cause the optical power transfer. Typically, evanescent field coupling is achieved by removing a portion of the cladding from the fibers, and positioning them in close proximity so that each core is within the evanescent field of the other core, thereby causing the aforementioned modal interaction.
One type of evanescent field coupler is described in an article entitled "Single Mode Fiber Optical Power Divider: Encapsulated Etching Technique", published in the January, 1979 issue of Optics Letters. This coupler, which will be referred to as a "bottle coupler", comprises two optical fibers that have been twisted together. The twisted fiber is inserted into a glass bottle which is filled with an etching solution to remove a portion of the cladding. The etching solution is then drained, and replaced by index matching oil. However, as indicated in the last paragraph of this article, this device has substantial throughput losses of about 2 dB. Further, it appears that this coupler may be mechanically unstable and fragile.
Other types of evanescent field couplers include integrated-optic couplers which comprise waveguides fabricated in planar substrates. However, these couplers have internal losses, relatively high input and output coupling losses and internal losses when used in fiber optic systems, and they tend to be sensitive to the polarization of the light being transmitted. Such a coupler is disclosed in an article entitled "Optical Directional Couplers with Variable Spacing", published in the Mar. 1, 1978 issue of Applied Optics.